Project Summary The paraventricular nucleus of the hypothalamus (PVH) is a brain region critical for energy homeostasis. It is composed of diverse cell populations that project throughout the brain, including to hindbrain areas known to regulate feeding and energy expenditure, such as the parabrachial nucleus (PBN) and the nucleus of the solitary tract (NTS). Receptors for the anorectic peptide, salmon calcitonin (sCT) are expressed in the PVH and direct injection of sCT to this nucleus dramatically suppresses feeding. Thus, we hypothesize that calcitonin receptor (CalcR)-expressing PVH neurons (CalcRPVH) contribute to energy homeostasis through discrete projections to specific brain regions. To determine the role of CalcRPVH neurons in energy balance, we generated a CalcR-2a-Cre transgenic mouse in order to selectively manipulate CalcR-expressing cells. We find that CalcRPVH neurons send dense projections to the PBN, NTS, and the median eminence. Moreover, chronic silencing of CalcRPVH neuronal activity leads to hyperphagic obesity indicating that CalcRPVH neuron activity is required for body weight maintenance. The objective of this proposal is to determine the cellular mechanisms and neural circuitry by which CalcRPVH neurons modulate energy balance. We hypothesize that activation of CalcRPVH neurons suppresses feeding and increases energy expenditure and that the feeding effects (but not energy expenditure effects) are mediated through projections to the PBN. In contrast, we also hypothesize that PVH to NTS projections will modulate both feeding and energy expenditure. In Aim 1 we will use chemogenetics and optogenetics to acutely activate CalcRPVH neurons (or their PBN projection terminals) and determine whether activation of these neurons and their terminals influence feeding behavior and/or energy expenditure. If CalcRPVH activation suppresses feeding (as our preliminary data suggests), we will determine whether this effect is aversive or non-aversive. In Aim 2, we will use retrograde viral techniques to delineate the contribution of PVH?PBN vs. PVH?NTS projecting cells in the regulation of energy homeostasis. By limiting our manipulations to subpopulations of PVH neurons based on their projection targets, we will test the hypothesis that these circuits differentially modulate feeding and energy expenditure. These studies will yield novel findings regarding the regulation of energy balance and identify novel potential therapeutic targets for the prevention and treatment of obesity. The advanced molecular methods, neurosurgical techniques and data analysis required to complete these projects will provide critical experience and skills for enhancing the applicant's development as an independent research scientist.